Voicemail isn't something Android controls. Your carrier hosts the voicemail service and keeps a copy of recorded messages. Carriers usually offer automated access to control these settings (check the SIM toolkit) but they're in control of the services they provide to you.
Android can try, but it's powerless if your carrier doesn't implement the right magical dial codes.
I don't know why Google doesn't offer settings for the number of rings before automatically declining a call. Other dialer apps I've seen have offered that. Your carrier probably offers that functionality as well.